Process for producing a low density foamed polyethylene

ABSTRACT

A foamed and expanded low density polyethylene sheet or tube possesses both superior buoyancy and thermal resistance characteristics. The sheet or tube is made by a process comprising mixing a low density polyethylene with a blowing agent, surface activation agent and, preferably, a separation agent, heating the mixture at different temperatures to first melt the mixture and then gasify the blowing agent to expand the polyethylene cells, cooling the melted mass to partially harden and shrink the expanded cells, further expanding the cells by introducing a second gaseous blowing agent, cutting, extruding and cooling the expanded mass, forming a sheet or tube from the cooling mass, and finally cooling the sheet or tube for at least 24 hours. A buoyant, thermal resistant, water-repellent and moisture resistant sheet or tube results, which can be used, for example, as a liner for garments, including sports, business and military garments and other related equipment.

TECHNICAL FIELD

This invention relates to a low density, expanded and foamedpolyethylene sheet or tube having excellent bouyancy, cold-resistant andmoisture-resistant properties, to a process for producing such a lowdensity foamed polyethylene, and to garments and equipment requiringboth bouyant and thermal resistant properties.

BACKGROUND OF THE INVENTION

Garments possessing buoyancy, cold-resistant and waterproof propertiesare highly desirable in a multitude of situations. Military personnelequipped with uniforms and jackets having all of these properties wouldfind them advantageous in a number of situations. For example, asoldier's maneuverability in rough terrain under adverse conditionswould be greatly enhanced by clothing which was both cold-resistant toprotect the wearer from the elements and simultaneously facilitatedcrossing a river or fording a stream due to its buoyant and waterproofproperties. Mountain climbers and hikers would find such garmentssimilarly advantageous, where crossing a river, dealing with a flood, orresisting the cold are all potential encounters. To achieve theirgreatest utility, such garments must not only possess these variousproperties, but they must also be comfortable and not bulky orcumbersome.

Water sports enthusiasts would also find such garments of particularadvantage. Indeed, for many activities, garments possessing buoyant,cold-resistant and waterproof properties would be ideal and find greatutility.

It is known to provide various garments with foamed materials forthermal insulation. Thus, U.S. Pat. No. 2,976,539 discloses athermally-insulated garment having an expanded, closed-cell cellularmaterial as a lining, preferably polyvinyl chloride. The garment isthick and cumbersome and does not possess sufficient buoyancy to supporta person's weight in water. Similarly, U. S. Pat. No. 3,511,743discloses a thermal insulation laminate for space and diving suits whosecore is an open or closed-cell sponge or foam. A fluid impermeablerubber or plastic skin is necessary. The laminate gives moderatemobility, but is still fairly cumbersome and does not have sufficientbuoyancy to serve as a life-saving device.

Polyethylene foams have been suggested as insulation or fillers for liferafts and jackets. U. S. Pat. No. 3,067,147 suggests a low densitypolyethylene foam for such a purpose which is processed with1,2-dichlorotetrafluoroethene as a foaming or blowing agent. Thepolyethylene and blowing agent are heated under pressure and explosivelyextruded into the atmosphere to form an expanded cellular mass. Theproduct is bulky and cumbersome when used in practice. Similarly, U.S.Pat. No. 3,819,543 discloses a molded chlorinated, cross-linkedpolyethylene foam for use in producing floats and linings for clothing.Various known blowing and cross-linking agents are suggested for use bythis reference. The molded products are thick and cumbersome.

Despite these advances, there remains a need for a thin, lightweightmaterial for use as a liner for garment and outdoor equipment whichpossesses a combination of superior buoyancy, cold-resistant andwater-resistant properties, without hampering the mobility of thewearer.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate theabove-mentioned drawbacks by providing a foamed, expanded low densitypolyethylene possessing both superior buoyancy and thermal resistancecharacteristics.

Another object of the present invention is to provide a process forproducing a foamed, expanded low density polyethylene possessing bothsuperior buoyancy and thermal resistance characteristics.

Yet another object of the invention is to provide a thin and lightweightlining for garments and sports, business, military and other relatedequipment which is not cumbersome and bulky, yet combines both superiorbuoyancy and thermal resistance characteristics.

These and other objects are attained in accordance with the presentinvention by a process comprising mixing low density polyethylene with ablowing agent, a surface activation agent and, preferably, a separationagent, heating the resultant mixture to form a softened mass, raisingthe temperature of the softened mass to gasify the blowing agent forfoaming and expanding the cells of the polyethylene, reducing thetemperature of the foamed and expanded polyethylene to partially shrinkand harden the polyethylene cells, introducing a gaseous blowing agentinto the polyethylene mass to cause additional foaming and expansion ofthe polyethylene cells, cooling the mass to a temperature suitable forcutting, cutting and heating the mass to a temperature suitable forextruding, extruding the mass, forming the mass into a sheet or tube,and cooling the sheet or tube for at least 24 hours at room temperatureto form a low density polyethylene sheet or tube characterized bysuperior buoyancy and thermal resistance.

The present invention further comprises a foamed and expanded lowdensity polyethylene sheet or tube produced in accordance with theprocess of the present invention.

The present invention further comprises a garment incorporating thereina lining of foamed and expanded low density polyethylene having athickness ranging from about 0.5 mm to about 1 mm and a buoyancy suchthat about 300 g of the lining will keep more than about 130 kg ofweight afloat.

In a preferred embodiment of the process according to the invention,azodicarbonamide is the blowing agent, zinc oxide is the surfaceactivation agent, ZnC is the separation agent, freon is the gaseousblowing agent and a foamed and expanded low density polyethylene sheetis formed to be used as a liner in garments requiring buoyant,water-repellent and thermal resistant characteristics.

The present invention will be better understood through the followingdetailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a low density polyethyleneprepared by conventional process is mixed, for example in a hopper, witha first blowing agent, a surface activation agent and, preferably, aseparation agent to form a dry mixture. The dry mixture is conveyed to aclosed heat tunnel, where it is processed in a series of stages atdifferent temperatures. A conventional thermocouple control box can beused to maintain a particular required temperature in each of theprocessing stages in the heat tunnel.

A conventional coil or screw conveys the mixture through a first portionof the heat tunnel. A pressure box at the entry end of the heat tunnelprovides pressurized air to assist in conveying the mixture through theheat tunnel.

In the first stage of the heat tunnel, the mixture is heated at atemperature of, for example, about l70°C. to form a melted and softenedpolyethylene mass. In the second stage of the heat tunnel, the meltedmass is heated at a temperature, depending on the gasifying temperatureof the blowing agent, sufficient to gasify the blowing agent and furthermelt the mass. The blowing agent and air begin to penetrate the cells ofthe melted low density polyethylene mass and cell expansion occurs.Gasifying of certain blowing agents releases nitrogen and carbonmonoxide, which react to form carbon dioxide. The carbon dioxide thenpenetrates the cells of the low density polyethylene, causing foamingand expansion of the cells, i.e. blowing.

In the third stage of the heat tunnel, the expanded polyethylene istreated at a temperature of, for example, about 150° C. to partiallyshrink and harden the expanded cells of the mass.

A low density polyethylene (LDPE) is well-known in the art and ischaracterized by a density ranging from about 10 to about 40 lbs/ft³ anda density ratio of about 0.910 to about0.925 as compared to water, wherewater is considered to have a density of 1. Higher density polyethylenesare not suitable for use in the invention.

Suitable blowing agents to be initially mixed with the low densitypolyethylene, preferably in powder form, include azodicarbonamide(commercially-available as Celogen AZ Kempore, gasifying temperature ofabout 195° C.), N,N'-dinitrosopentamethylene-tetramine(commercially-available as Unicel NDX, gasifying temperature of about195 ° C.), and 4,4' Oxybis (commercially-available as Celogen OT,gasifying temperature of about 150° C.). The amount of blowing agentadded is substantially about 1% by weight of the polyethylene. Amountsof blowing agent substantially greater than about 1% are not useful forpurposes of the present invention because an overexpansion of thepolyethylene cells takes place. Similarly, amounts of blowing agentsubstantially less than 1% do not permit sufficient cell expansion forpurposes of the present invention.

Azodicarbonamide is the preferred initial blowing agent, especially whenmaking linings for garments, sleeping bags and other articles worn orused intimately. It is odorless, non-toxic, causes no discoloration ofthe final product and creates a maximum amount of gas for blowing.Comparatively, azodicarbonamide yields about 220 cc of gas per gram ofsolid, whereas 4,4' oxybis yields only 130 cc/g andN,N'-dintrosopentamethylene-tetramine 210 cc/g.

Suitable surface activation agents to be initially mixed with the lowdensity polyethylene, preferably in powder form, include zinc oxide,cadmium oxide and calcium carbonate. The surface activation agent isadded in an amount ranging from about 0.1 to about 0.2% by weight of thepolyethylene.

The surface activation agent performs several important functions.First, it activates the blowing process while preventing too rapid anexpansion of the LDPE cells during blowing. Secondly, it keeps thetemperature in the heating tunnel down during the blowing process. Forexample, where azodicarbonamide is used as the blowing agent, thesurface activation agent assists in maintaining a temperature of around150° C. in the third stage of the heating tunnel. Absent this agent, thegasified blowing agent would raise the temperature to around 196° C.,causing excessive blowing not contemplated by the present invention.

It is preferred, but not essential, to add a separating agent to beinitially mixed with the low density polyethylene is ZnC. The separatingagent, added preferably in powder form, aids in preventing the LDPE fromsticking to the coil or walls of the heating tunnel. The amount ofseparating agent added is substantially about 0.3% by weight of thepolyethylene.

In a fourth stage of the heating tunnel, a gaseous blowing agent isintroduced into the polyethylene mass to subject the mass to a secondphase of cell expansion and strengthen the bubbled cells of the mass.This second blowing agent is introduced at a pressure of about 35kg/cm².

The preferred gaseous blowing agent is Freon, the well-known trademarkfor dichlorofluoromethane, although liquid petroleum gas (LPG) or amixture of freon and liquid petroleum gas may be used. Other suitablegaseous blowing agents include monochlorodifluoromethane anddichlorodifluoroethane. The advantage to using LPG is itsinexpensiveness, although it has potentially toxic effects. Accordingly,in making an expanded polyethylene for linings of garments, sleepingbags, etc., it is preferable to use blowing agents having no potentiallytoxic effects.

After the introduction of the gaseous blowing agent and additionalexpansion of the polyethylene mass, the mass is subjected in a fifthstage of the heating tunnel to a temperature of about 100° C., whereinit begins to cool. Treatment of about 100° C. prepares the mass forproper and efficient cutting. This completes the heating and blowingprocess.

The first through fifth stages in the heating tunnel take from about 30minutes to about 1 hour to complete, each stage taking approximately thesame amount of time.

The expanded mass continues its travel through a second portion of theheating tunnel which does not have any coil or screw therein.

In this second portion of the heating tunnel, a desired quantity of theexpanded mass is cut, for example, by a conventional cutting blade. Theexpanded mass is still in a softened state. The desired quantitydepends, of course, on the size of the final sheet or tube desired.

The cut, expanded mass is next subjected to a temperature of about 105°C. in the heating tunnel and extruded through a die and mandrel into afree expansion zone and cooling zone at atmospheric pressure and roomtemperature. The temperature of the mass must be raised slightly aftercutting because a temperature of about 100° C. is too cool for properextruding. After extrusion, the foamed polyethylene mass expandsnaturally in the atmosphere, but not explosively, and cools at roomtemperature for a short period, e.g., a few seconds. The cooling mass ofpolyethylene is then formed into a sheet or tube by conventional means,the thickness or diameter, respectively, being determined by the desiredend use of the product.

The sheet or tube can then be wound on rolls, after which it is cooledat room temperature (20° C. to 30° C., preferably 25° C.) for at least24 hours. The sheet or tube goes through two stages during this 24 hourcooling period. Initially, the cells of the expanded mass collapse asentrapped blowing agent and air work their way out of the cells.Subsequently, over the 24 hour period, air repenetrates the cells toexpand them to an intermediate stage.

The final product is a low density polyethylene in which innumerablesmall cells containing air form a close-packed network. The weight ofthe final product corresponds to the weight of the initial low densitypolyethylene. Thus, for example, 25 kg of low density polyethylene yield25 kg of finished sheeting.

The present invention thus provides a foamed, expanded low density sheetor tube of polyethylene of the formula --CH2--CH2-- having a weightranging from about 12 g/yd² to about 21 g/yd² for sheets of thicknessesranging from about 0.5 mm to about 1 mm. The sheet is characterized bysuperior buoyancy and thermal-resistant properties. For example, 300grams of the finished product can float more than 130 kg of weight. Thedensity ratio of the final product ranges from about 0.4 to about 0.55as compared to water, where water is considered to have a density of 1.Additionally, the sheet or tube of low density expanded and foamedpolyethylene is cold-resistant, water-repellent and moisture resistant.

The expanded and foamed low density polyethylene of the presentinvention can be formed into a sheet having any thickness, butpreferably a thickness ranging from about 0.5 mm to about 5 mm. Forgarments, the thickness of the sheet should be no less than about 0.5 mmand no more than about 1 mm. Sheets having thicknesses much less than0.5 mm do not possess sufficient buoyancy to be practical for use in theinvention. Conversely, sheets having thicknesses much greater than about1 mm are not suitable for use in garments of the present invention,because they make for an uncomfortable garment. Where sheets of lowdensity polyethylene according to the present invention are intended foruse as liners in tents, sleeping bags, blankets and similar equipment,thicknesses up to about 5 mm may be advantageously used without makingthe equipment unduly bulky and cumbersome.

During the mixing step, conventional additives can be added inappropriate amounts to impart additional characteristics to the finalproduct, such as a fireproofing or anti-inflammatory agent such as tin.These conventional additivies increase the weight of the final productslightly, but do not significantly affect the buoyancy or thermalcharacteristics of the final product. Exemplary additives include about0.1% to about 0.2% by weight of the polyethylene of crosslinking agentssuch as azobisformamide (ABFA) or dicumyl peroxide, which can be addedin powder form to increase the resistance of the final product totearing, as well as about 0.1% of an ultra-violet absorber to preventdiscoloration. After the foamed and expanded polyethylene is formed intosheets, or simultaneous with the sheet-forming step, the polyethylenemay be sandwiched between opposed sheets of a material such as a nylon,vinyl, plastic, gauze, sheer fabric, or any material covering. Since thepolyethylene is still in a somewhat softened state at this point, thesandwiching material adheres slightly to the polyethylene. Moreimportantly, sandwiching at this point facilitates the process of sewinga material cover to the polyethylene, as well as cutting andtransporting of the final product. Additionally, such a material coverprolongs the durability of the sheet.

No special conditions are required for storage of the low density foamedpolyethylene produced in accordance with the invention, even during thefinal 24 hour cooling period, in view of the water-repellent andmoisture-resistant properties of the product.

Additionally, there is no waste of polyethylene in the process. Forexample, 10 grams of low density polyethylene starting material yields10 grams of expanded and foamed low density polyethylene produced inaccordance with the invention.

The diameter of the polyethylene tubes produced in accordance with thepresent invention varies according to the end use. The extruded tubesmay be solid or hollow and find particular use in providing buoyantmaterial around boats.

The following example illustrates an application of the principles ofthe present invention. However, it is not intended and should not beconstrued as limiting the scope of the invention.

EXAMPLE

25 kg of low density polyethylene are mixed in a hopper with 250 g ofazodicarbonamide, 40 g of zinc oxide, and 75 g of ZnC. The mix is fedinto a heating tunnel containing a helical screw which feeds the mixthrough the tunnel. Air fed from a pressure box assists in conveying themix. The mix is heated at 170° C. in a first stage of the tunnel whereit melts. The melted mass travels to a second stage of the tunnel,maintained at a temperature of 200° C., where the azodicarbonamidegasifies and penetrates and expands the polyethylene cells. The mass isthen subjected to a temperature of 150° C. in a third stage to cool themass, whereby the cells harden and shrink slightly. Freon is thenintroduced at 35 kg/cm² to further expand and foam the mass. Thetemperature of the mass is next lowered to 100° C. to condition the massfor cutting. The mass is cut with a cutting blade such that it seversfrom the next batch following it through the tunnel. The temperature ofthe mass is raised to 105° C. to prepare the mass for extrusion. Themass is extruded into an ambient atmosphere at a room temperature of 25°C. Expansion takes place, but the mass does not explode. The extrudedmass cools for a few seconds and then passes through rollers to form asheet having a thickness of 1 mm. The continuous sheet is formed into 4rolls and stored at 25° C. for 24 hours. The final product is 4 rolls ofexpanded and foamed polyethylene having a thickness of 1 mm, with 300 mto the roll, a weight of 21 g/yd², a thermal transmittance of 6.70 w/m²C and a thermal transmission of 52.5% (ASTMD 1518) and a waterabsorption of 9.1×10⁻⁴ lb/ft² (BS 3595-1969). 300g of the finished sheetkeeps more than 130 kg of body weight afloat.

The foamed and expanded low density polyethylene of the presentinvention finds a multitude of uses as linings for garments, sleepingbags, blankets, cushions and sports and leisure equipment such as tents,bags, backpacks, rafts and deck chairs and other related items. Sincethe low density polyethylene product of the present invention combinesboth superior flotation ability and thermal resistance, the need todispatch additional safety gear on the water is eliminated. A singlegarment incorporating the lining of the present invention serves as botha thermal jacket against the elements and a flotation device for generalflotation and emergencies. Morever, the present invention provides alightweight and thin garment for the above purposes which is comfortableand not cumbersome, thus improving enjoyment as well as safety.

The present invention provides a lining and garment which retains itsbuoyancy and flotation ability even when ripped or torn. Also, blanketsand sleeping bags can be provided with buoyant properties to serveadditionally as flotation devices. The cumbersome safety vest used, forexample, by water skiers can be replaced by a thin and lightweightsafety vest, thus improving safety and maneuverability in leisuresports.

Conventional hunting, fishing, camping and military clothing and relatedequipment can be dramatically improved by the instant invention,resulting, for example, in a single garment that can be worn in thecountryside which is thermally insulated and immediately usable inemergency flotation situations varying from accidentally falling into ariver to the need to ford a stream. Application of the present inventionto military uniforms, jackets and ponchos morever extends a soldier'scapabilities in battle situations. The polyethylene product of thepresent invention can also be used as thermal insulation, for example,for military radio equipment and the like.

It will be appreciated that while the present invention has beendescribed with reference to specific and preferred embodiments thereof,this is not done by way of limitation, and various modifications willsuggest themselves to those of ordinary skill in the art which fallwithin the spirit and scope of the present invention as set forth in theappended claims.

What is claimed is:
 1. A low density article possessing excellentbuoyancy, thermal-resistance and water repellency comprising a flexible,expanded, closed-cell polyethylene sheet having a thickness of 0.5 to1.0 mm, a density ranging from about 0.0251 to 0.0285 g/cm³, a buoyancysuch that 300g of the polyethylene keep more than about 130kg of weightafloat, a water absorption of about 0.0041 g/cm³ for a thickness of thepolyethylene and a thermal transmittance of about 6.70 w/m² C and athermal transmission of about 52.5% for a 1mm thickness of thepolyethylene.
 2. A low density article as in claim 1, wherein thepolyethylene is formed as a sheet or tube.
 3. A low density article asin claim 1, wherein the polyethylene contains innumerable, fineair-containing cells forming a close-packed network of closed cellswithin the polyethylene.